Much effort has been made in recent years to develop low power color displays. Such efforts have generally employed LCD panels in one of two configurations. In the first configuration, a plurality of differently colored LCD panels are stacked and illuminated with white light. As the light passes through the stacked layers, pixels in each panel act as controllable color filters, selectively coloring the light exiting the display.
While the stacked cell approach can provide a full color display, it has certain shortcomings. One is the requirement of three LCD panels. This large number both increases the cost of the display and reduces its light transmissivity. Furthermore, the stacking of cells often introduces serious parallax problems which severely limit the usable viewing angle.
The second approach uses only a single LCD panel, but uses it in conjunction with a mosaic color filter. The mosaic filter typically has a plurality of red, green and blue filter elements, each aligned with a pixel in the LCD panel. (The filter is often fabricated as an integral part of the LCD assembly.) By controlling the pixels in the LCD panel, the display can pass light through selected areas of the color mosaic filter.
While the color mosaic technique addresses certain shortcomings of the stacked panel approach, it introduces certain problems of its own. One is that brightness is limited because less than a third of the active area transmits light for any given color. Another is that pixel density must be increased by a factor of three to obtain the same resolution as the stacked cell approach. That is, to provide a color display with a horizontal resolution of 640 colored pixels, for example, the LCD panels must have 1920 pixels, 640 each of red, green and blue. This introduces fabrication problems that decrease yields and increase panel costs.
To address these shortcomings of the prior art, the present invention adopts a hybrid approach in which both stacked panels and color mosaic filters are employed. Preferably, only two stacked panels are employed, minimizing the problems conventionally associated with three panel embodiments. Similarly, only two color filter elements are used in the color mosaic filter, again minimizing the problems conventionally associated with the color mosaic approach. The filter colors are desirably selected from the subtractive primaries (cyan, magenta, yellow) so as to increase light transmission. The filter colors and the colors of the two LCD panels are selected so that the two technologies cooperate to provide a full color display.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.